Inflammatory bowel diseases (IBD), including ulcerative colitis (UC) and Crohn's disease (CD), are a group of chronic inflammatory disorders that affect individuals throughout life. Several studies have indicated that dysregulated host/enteric bacterial interactions are required for the development of chronic intestinal inflammation. However, the exact mechanisms underlining the initial host/microbial interaction in triggering colitis and the following steps for exacerbating colitis have not been fully defined. We have recently identified a novel, intestinal inflammation-associated inducible molecule Chitinase 3-like-1 (CHI3L1) that is produced by colonic epithelial cells (CECs) and lamina propria cells. CHI3L1 may be involved in the pathogenesis of colitis by enhancing intracellular bacterial adhesion and internalization on/into CECs. A major goal of this study is to define the biological significance of CHI3L1 in the pathogenesis of colitis. In Aim-I, we will examine the in vivo biological function of CHI3L1 in the pathogenesis of murine models of acute and chronic colitis actively in vivo through administration of the specific antibody. In Aim-II, we will test the interaction between microorganism-derived chitin-binding proteins and CEC-derived CHI3L1 in the adhesion and invasion of microorganisms to CECs. In Aim-III, we will define a prophylactic effect of orally administered chitin, a polymer of N-acetylglucosamine and substrate for CHI3L1, in animal models of acute and chronic colitis. These studies will help clarify the critical role of CHI3L1 in inflammation and provide a rationale for the development of novel anti- CHI3L1 based immuno-therapeutics, as well as chitin sensitization-based prophylactic approaches in IBD. We believe that the data obtained from this study would provide an important clue for understanding the role of CHI3L1 in the pathogenesis of IBD.